A varactor is an electrical device having a capacitance which is controlled by a suitable voltage bias. A varactor is used, for example, in Voltage Controlled Oscillators (VCOs), where a frequency of an oscillator is controlled by an applied voltage or current bias. VCOs are used, for example, when a variable frequency is required or when a signal needs to be synchronized to a reference signal. In radio communication devices, e.g. portable/cellular phones, VCOs are often used in Phase Locked Loop (PLL), circuits to generate suitable signals. Varactors are also useful in other circuits, such as tunable filter circuits, where the variable capacitance can be used to adjust the frequency characteristics of the filer circuit.